Negative Values in Heatmap

Support for the heatmap to display colors based on magnitude (absolute value) of values would be appreciated. I forked this repository to add this type of support in the "ratio" variable if values have negative and positive values.

Probably need a more generalizable implementation than the one I hacked together since it only works if you want to center the ratio such that the minimum is 0.

Review the heatmap React component and the ratio variable handling described in the issue. Determine how colors should represent values by magnitude when positive and negative values coexist, then verify the behavior for both mixed-sign and non-negative data.

Written by the indexing model from the issue text.